The field of the disclosure relates generally to combustion of a vanadium-contaminated liquid fuel in a gas turbine combustion system, and more particularly to protecting against vanadium corrosion and reducing the fouling of the hot parts of such gas turbines, while preventing any erosion phenomena, during operation.
The “hot parts” of a gas turbine are those of its components which are in contact with the combustion gas. In known gas turbines, these combustion gases have a speed of several hundred meters per second and temperatures exceeding 1000° C. The hot parts, which are made of super-alloys (in general, nickel-based) and can be covered with ceramic coatings (for example, anti-corrosion coatings and/or thermal barriers), typically include the components of the combustion system (such as combustion liners, transition pieces, and the like) and, downstream in the direction of the flow of the combustion gas, the static (“partition vanes” or “nozzles”) and rotating (“buckets” or “blades”) components of the expansion turbine.
When a gas turbine burns petroleum fractions of degraded qualities, such as heavy oils, distillation residues and other refining by-products (e.g. HCO), its hot parts can be subjected to three main types of damage: hot corrosion, fouling due to the formation of deposits, and erosion. With regard to corrosion, the oxidation of the vanadyl-porphyrins that are present in these fuels generate ash that is corrosive towards the materials of the hot parts. This form of corrosion, called “vanadic corrosion,” is caused by the formation in the flames of vanadium compounds of degree of oxidation (5) having low melting point (Tf), such as vanadium pentoxide (V2O5: Tf=675° C.) in free form or combined with alkaline metals such as alkaline meta-vanadate (NaVO3: Tf=628° C.; KVO3: Tf=517° C.). These compounds are transported in liquid state by the combustion gas from the combustion system towards the turbine, and the fraction which is deposited on the hot parts can cause severe electrochemical attacks that are characteristic of molten electrolytic media. When the gas turbine operates at full regime, the thermal environment of the hottest parts, typically the first stage nozzles of the turbine, is characterized by the “firing temperature,” which does not refer to the temperature existing in the flames but designates that of the combustion gases at their entry in the expansion turbine; it represents a particularly important design parameter of the gas turbines as it determines its maximum efficiency.
The vanadic corrosion can be inhibited by chemically trapping V2O5 within refractory and chemically stable compounds which suppress the molten electrolytic medium and, therefore, this form of high temperature corrosion. In this regard, magnesium salts constitute very good inhibitors. When injected in the combustion chamber, they react with the vanadium compounds to form magnesium orthovanadate, Mg3V2O8, whose melting point is 1170° C., according to the reaction:3MgO+V2O5→Mg3V2O8  (1)
The inhibitor must be injected in sufficient quantity for, on the one hand, trapping all the vanadium brought by the fuel and, on the other hand, forming the orthovanadate rather than a vanadate less rich in magnesium, such as the pyrovanadate Mg2V2O7 (Tf: 980° C.) or the meta-vanadate MgV2O6 (Tf: 742° C.), which are less refractory than the orthovanadate. The minimum theoretical value of the molar ratio (MgO/V2O5) ensuring the formation of orthovanadate Mg3V2O8 is 3. However, an excess of magnesium is required to obtain a good anti-corrosion protection. The inhibition reaction is in general written as:mMgO+V2O5→Mg3V2O8+(m−3)MgO  (2a)
In this equation, “m” refers to the molar ratio (MgO/V2O5) and will also be called “dosage ratio”; the excess of magnesium, defined as “e,” amounts to:e=m−3.  (3)
Thus, the equation (2a) can also be written:(3+e)MgO+V2O5→Mg3V2O8+eMgO  (2b)
In practice a high value of excess magnesium, and consequently a high value of the dosage ratio “m,” is necessary for, on the one hand, strictly guaranteeing the reliability of the anti-corrosion protection, and on the other hand, forcing the formation of Mg3V2O8 and reducing that of Mg2V2O7. Thus, one requires m=12.6 moles of MgO (instead of 3 theoretically) per mole of V2O5, or in weight terms, 3 grams of Mg per gram of V (i.e., an Mg/V ratio equal to 3 by weight). The equation of the corresponding inhibition reaction, which will be called the “conventional inhibition” or “conventional method” of inhibition, is written as:12.6MgO+V2O5→Mg3V2O8+9.6MgO  (2c)
The problem of fouling of the hot parts of the gas turbines by deposits of Mg3V2O8 and MgO, as well as the “dry cleaning” methods by injection of friable particles and the “wet methods” based on washing of the turbine with water, are described in European Patent No. 2,236,585, issued May 16, 2012, the disclosure of which is incorporated herein by reference in its entirety. The physical and chemical change of these deposits is complicated by the fact that a balance of sulfation/desulfation of MgO is established at high temperature, according to the reversible equation (4):MgSO4→MgO+SO3.  (4)
When the temperature rises, the magnesium sulfate, which is water soluble, tends to desulfate and be replaced with magnesium oxide, which is neither soluble in water nor in any chemical reagent compatible with the turbine materials. Moreover, this desulfation is accompanied by a physical contraction and an agglomeration of the deposit which tends to sinter and thus become more adhering and more difficult to dissolve and to disintegrate mechanically. Therefore, when the firing temperature of the turbine is increased, the high magnesium excess used in the conventional method leads to an increased fouling by MgO and increased difficulty in cleaning the hot parts. In practice, when the temperature exceeds approximately 1090° C., the deposits of magnesium-vanadium ash can no longer be removed from the hot parts, either by dry cleaning (an operation carried out “on-line”), nor by washing with water (an operation carried out “off-line”). This temperature level of 1090° C. represents a barrier for the efficiency of gas turbines burning fuels contaminated with vanadium and inhibited with magnesium, even though known gas turbines can operate at firing temperatures lying between 1140° C. (second generation machines or “E class”) and 1430° C. (third generation machines or “F class”) when the fuel is very pure. Moreover, because the power decreases in a virtually linear manner with the rate of fouling of the turbine, there is an obvious interest in reducing the deposition of ash even when the temperature is maintained at 1090° C.
Considering the limits of the inhibition methods which have been set out, it is desirable to provide new methods that: (i) ensure anti-corrosion protection at least as effective as that provided by the conventional method; (ii) generate minimum quantities of deposits which must moreover be easily removable, preferably according to an “on line” process that does not degrade the availability of the machine; and (iii) do not cause an issue regarding erosion of the hot parts. These three criteria, which must be satisfied up to the highest possible limit temperature to optimize the efficiency, constitute the “advanced inhibition objective.”